DESCRIPTION (Investigator's Abstract): New techniques for three- dimensional motion analysis are utilized to analyze the mechanisms of two complementary disorders affecting the control of movement that until now have been studied primarily only with conventional means: patients with left-hemisphere lesions and limb apraxia, who show deficits in planning complex motor acts, and patients with right hemisphere lesions and neglect, who show deficits in motor intention, spatial attention, and in the representation of extra-personal space. Differential deficits in the organization of reaching movements is investigated, experimentally manipulating requirements for movement planning, sensorimotor transformations, and movement execution. Patterns of pointing errors in 3D space, distortions in 3D hand trajectories, and deficits in inter-joint coordination during the movements are analyzed. Experiments utilize a programmable robot arm to present targets in known locations in one of two planes in space. Spatial Perception Thresholds. Pure perceptual errors will be measured by having subjects visually discriminate actual target locations from nearby locations presented I-I sec earlier and then extinguished. Pointing to Actual and Memorized Targets With and Without Visual Control. Disturbances in pointing movements are investigated when subjects use different conditions of visual feedback: no visual feedback during the movement, vision of the arm but not of the target, or vision of the arm and the target. Deficits in sensorimotor transformations, trajectory planning, and in spatial attention underlying action will be investigated. Pointing to Memorized Targets Under Conditions of Extreme Head Rotations. The degree to which hemispatial neglect is governed by head or body position relative to external space is investigated by having subjects turn (heir heads to the extreme right or to the extreme left after target presentation but before pointing is initiated. Right-lesioned subjects with neglect may show an asymmetric contraction of space when pointing with their heads rotated to the extreme right. Planning Trajectories to Avoid an Obstacle. Deficits in motor planning will be investigated in experiments in which subjects point to remembered target location with and without visual feedback, in the presence of an obstacle. The position of the obstacle with respect to the initial arm position and target location will be changed systematically. Right-lesioned subjects may exhibit specific distortions in the relation between the two subcomponents of the movement pointing itself and avoiding the obstacle. Performance of left-lesioned apraxic subjects on the pointing tasks will allow insight into the degree to whim left hemisphere movement control systems specifically mediate learned, skilled movement versus kinematic planning and sensorimotor transformations for movement in general.